Check-raise
A check-raise is checking with the intention of raising once your opponent bets, letting the out-of-position player seize back the betting initiative instead of only calling or folding.
A check-raise only happens out of position: you act first, check, and if your opponent bets you raise instead of calling or folding. The trap only springs if they take the bait, so it depends on facing an opponent willing to bet into a check. Done well it builds a bigger pot with value hands and denies equity to hands that would rather see a cheap card, and it punishes an aggressor who bets too wide out of habit.
Worked example. The pot is 20bb on the flop. Your opponent bets 15bb, taking the pot to 35bb, and you raise to 50bb total. To continue, they must call a further 35bb into an 85bb pot:
pot odds to continue = 35 / (35 + 85) = 0.292
Your raise changes the price of their decision from a cheap continue against a single bet to a call that needs almost 30% equity, which is why a credible check-raise threat keeps an aggressor honest.
A range of only the nuts is easy to play against, so mix in semi-bluffs alongside your value hands to stay unpredictable.